The phrase wouldn’t say boo to a goose is used to emphasise that someone is very timid. Here, boo is not an expression of disapproval but a later form of bo, an exclamation intended to frighten. In Notes and Queries of 10th September 1870, the English philologist Walter William Skeat (1835-1912) wrote:
“Cry Bo to a Goose”
This has not yet been quite explained, I think. To be able to say “Bo!” to a goose is to be not quite destitute of courage, to have an inkling of spirit, and was probably in the first instance used of children. A little boy who comes across some geese suddenly will find himself hissed at immediately, and a great demonstration of defiance made by them; but if he can pluck up heart to cry “bo” loudly, and advance upon them, they will retire defeated. The word “bo” is clearly selected for the sake of the explosiveness of its first letter, and the openness and loudness of its vowel.
Another word of the same character is “sho!” or “shoo!” considered useful in frightening cats.
In fact, the earliest printed version uses shoo, an exclamation used to drive away fowls or animals. It is found in A seconde admonition to the Parliament, an open manifesto of the Puritan party written in 1572, in which is evoked someone
who can scarse say (as they say) shue to a goose.
The parenthesis “as they say” indicates that the phrase was already proverbial at that time.
In Paroemiologia Anglo-Latina in usum scholarum concinnata. Or proverbs English, and Latine, methodically disposed according to the common-place heads, in Erasmus his adages (1639), the schoolmaster John Clarke (died 1658) recorded the following proverb:
He cannot say shooh to a goose.
The version with bo is first recorded in Oh read ouer D. Iohn Bridges, for it is a worthy worke: Or an epitome of the fyrste Booke of that right worshipfull volume, written against the Puritanes, in the defence of the noble cleargie, by as worshipfull a prieste, Iohn Bridges, Presbyter, Priest or elder, doctor of Diuillitie, and Deane of Sarum (1588), a religious pamphlet written under the pseudonym of Martin Mar-Prelate:
He is not able to say bo to a goose.
The Bishop of Norwich and religious controversialist Richard Montagu (1577-1641) used battledore instead of goose in Diatribæ upon the first part of the late history of tithes (1621); he wrote that “the Clergy of this time were” “not able to say bo to a battledore”.
In Lenten Stuffe (1599), the English pamphleteer Thomas Nashe (1567-circa 1601) used to say bo to in the sense of to gainsay:
(1871 edition)
All this may pass in the Queen’s peace, and no man say bo to it.
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